WO 97/28 513 discloses a system in which a digital image of a footprint is captured, displayed on a computer screen, cropped to a predetermined size, and the rectangular co-ordinates of any unique identification features are recorded and stored in a database. The stored co-ordinates can then be compared to identify any similar sets of co-ordinates which are likely to originate from the same article. In order to reduce registration errors care must be exercised in the positioning of the image. In addition, in order to take account of any registration errors multiple comparisons are made with the sets of co-ordinates being incremented or decremented between comparisons.
The present invention seeks to provide an inventive improvement on the earlier system.